1. Technical Field
The present invention relates to a cellulose acylate film, a polarizing plate and a liquid crystal display device.
2. Related Art
Films of polymers represented by cellulose acylates, polyesters, polycarbonates, cycloolefin polymers, vinyl polymers, polyimides and the like have been used in silver halide photographic light-sensitive materials, phase difference films, polarizing plates and image display devices. Films which are better in terms of planarity and uniformity may be produced from these polymers, so that these polymers have been widely employed as films for optical applications. For example, a cellulose acylate film having an appropriate water vapor permeability may be directly joined online to a polarizer composed of the most commonly used polyvinyl alcohol (PVA) and iodine. For this reason, a cellulose acylate, particularly, a cellulose acetate film has been widely employed as protective films for the polarizing plates.
When cellulose acylate films are used for the optical applications, laminated films including a core layer and a skin layer having different acyl substitution degrees have been recently developed from the viewpoint of improving various characteristics of cellulose acylate films.
For example, Japanese Patent Laid-Open Publication No. 2011-162769 (hereinafter JP-A-2011-162769) discloses a cellulose acylate film in which a dope A for an inner layer (core layer) having an acyl substitution degree of 2.43 and a dope B for a surface layer (skin layer) having an acyl substitution degree of 2.81 are laminated by co-casting.
Further, Japanese Patent Laid-Open Publication No. 2012-098646 (hereinafter JP-A-2012-098646) describes a phase difference film containing the same specific additive in a predetermined amount in a core layer and a skin layer as a phase difference film which includes a skin layer including a cellulose acylate having an acyl substitution degree of 2.90 on both surfaces of a core layer including a cellulose acylate having an acyl substitution degree of 2.20.
However, when the laminated film including a core layer and a skin layer described in JP-A-2011-162769 is applied to a polarizing plate, a liquid crystal display panel, and the like, there are some cases where the skin layer and the core layer are easily peeled off and the yield is reduced when the polarizing plate is cut off after the polarizer and the film are joined, when an edge face is polished, when the polarizing plate is re-joined to glass in the case where problems occur in the process of joining the polarizing plate to glass, or the like.
In addition, the phase difference film described in JP-A-2012-098646 is intended to enhance adhesion between layers, but has problems in that adhesion force is easily lacking, the kind and addition amount of additive are limited, and the like when stretching conditions are adjusted for the purpose of optical characteristics, and the like.
From those described above, an object of the present invention is to provide a cellulose acylate film including a core layer and a skin layer, in which the cellulose acylate film has a uniform surface shape and has excellent interlayer adhesion without being limited by additives, and the like, and a polarizing plate and a liquid display device, which include the cellulose acylate film. Furthermore, another object of the present invention is to provide a production method, which may produce the cellulose acylate film while maintaining a uniform surface shape without increasing the number of processes.
The present inventors have intensively studied, and as a result, have found that the above-described problems may be solved by forming, between the core layer and the skin layer having different average acyl substitution degrees from each other, a mixed layer which is mixed with cellulose acylate contained in both layers, in a specific thickness.
That is, the present invention has the following configuration.